The classic demonstration of volume thermal expansion - great for drawing and keeping the attention of your student(s).

Put the ball through the ring;

Heat the ball until it is very hot;

Try to bring the ball back through the ring. The ball cannot pass back through the ring until it has cooled!

Once you have their attention, try some other variations:

Heat the ring instead of the ball. Does the ball go through the ring now?

Heat the ring and the ball. Can it go through now?

Historically, the ball and ring experimental apparatus has been referred to as 'sGravesande Apparatus. Willem Jakob van 'sGravesande (1688-1742) wrote one of the first Newtonian physics textbooks in 1720-1721.

The original 'sGravesande ring is currently at the Universidade de Coimbra museum in Portugal. Look at a photograph of the 'sGravesande ring.